1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with an improved retention strap for a grid assembly fixture which permits assembling a grid having corner cells of proper squareness and size.
2. Description of the Prior Art
In most nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies. Conventional designs of these fuel assemblies include a plurality of fuel rods held in an organized array by a plurality of grids spaced axially along the fuel assembly length and attached to a plurality of elongated control rod guide thimbles of the fuel assembly. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the guide thimbles which extend slightly above and below the ends of the fuel rods.
The grids as well known in the art are used to precisely maintain the spacing between the fuel rods in the reactor core, prevent rod vibration, provide lateral support for the fuel rods, and, to some extent, frictionally retain the rods against longitudinal movement. Conventional designs of grids include a multiplicity of interleaved inner straps having an egg-crate configuration designed to form cells which individually accept the fuel rods and control rod guide thimbles. The cells of each grid which accept and support the fuel rods at a given axial location therealong typically use relatively resilient springs and relatively rigid protrusions (called dimples) formed into the metal of the interleaved straps. The springs and dimples of each grid cell frictionally engage or contact the respective fuel rod extending through the cell. Additionally, outer straps are attached together and peripherally enclose the inner straps to impart strength and rigidity to the grid. The outer straps conventionally have springs integrally formed into the metal thereof which project into respective ones of the cells disposed along the perimeter of the grid.
Over the years, the manufacture of a fuel assembly grid has been an intricate and time consuming operation. However, just recently a grid assembly fixture has been developed which has helped overall to improve grid manufacture by permitting relatively rapid assembly of the inner and outer grid straps without causing undue bending thereof. Such grid assembly fixture is described and illustrated in U.S. Pat. Nos. 4,492,844 and 4,519,593 to Richard M. Kobuck et al and assigned to the assignee of the present invention. The fixture is not only used to accurately assemble the grid straps to form the grid but also provides the means by which the straps of the assembled grid are retained together during subsequent welding thereof. The fixture includes a retention strap composed of four generally H-shaped bars, each bar including a cross member interconnecting a pair of end posts. The opposite end posts of the retention strap bars are connected together adjacent to the corners of the assembled grid so that the retention strap will extend about the periphery thereof and hold the grid straps in assembled relationship on the fixture. Each bar at its opposite end posts is respectively hingedly fastened and releasably fastened to the posts of the adjacent bars. The bars further have screws adjacent their respective posts for providing adjustment in the amount of an inwardly-directed clamping force applied to the assembled grid at the corners thereof by the retention strap. Also, pins (not shown in the referenced patents) are disposed through the corner grid cells, in engagement with the springs and dimples therein, for the purpose of maintaining the corner cell size, squareness and dimple perpendicularity.
Notwithstanding the overall benefits fostered by the grid assembly fixture of the aforesaid patents, the outer retention strap of the fixture has generally failed to maintain grid corner cell size, squareness and dimple perpendicularity specifications at the desired standards. The inwardly-directed clamping force applied by the retention strap to the grid corners is transferred via the pins against the dimples to the adjacent inner straps which contain the dimples and comprise the cell walls. Such force causes deflection and distortion of the cell inner walls. The outer walls of the corner cells also tend to distort inward during the corner seam or joint welding operation which results in an undersized cell condition. Both of these conditions require an additional repair operation to correct cell distortion in order for the grid to meet minimum envelope standards at the corner.
Consequently, a need exists for improvement of the above-described grid assembly fixture in a way which will enhance production efforts to hold desired grid cell size, dimple perpendicularity and corner squareness and thereby maintain minimum envelope requirements at the corners and eliminate the necessity for corrective measures.